The present invention relates to fuel cells which are supplied with liquid fuel, for example, a fuel cell which is suitable for size reduction as to become containable even in small-size electronic equipment such as portable telephones.
The fuel cell is an electric power generation device which generates electric power when supplied with fuel and oxidizer. Generally, since air may be used as the oxidizer, the fuel cell is capable of continuous power generation by replacement of the fuel. Therefore, the fuel cell has been drawing great attention also not only as stationary power supply but also as portable power supply.
Generally, in stationary fuel cells or the like, hydrogen or a gas containing hydrogen is used as the fuel. However, for the portable power supply, it is advantageous to be capable of generating electric power for longer time while the container having the fuel stored therein is unchanged. Therefore, as the fuel, liquid fuels higher in energy density per volume are more advantageous.
In addition, hydrogen generated from a liquid fuel by a reformer can be used for power generation. However, this causes the whole fuel cell system to become more complex, and therefore it is considered that the size reduction can be achieved more easily by direct supply of the liquid fuel.
Conventionally, a fuel cell of the direct fuel-supply type is disclosed in JP H11-510311 A. This fuel cell is a direct methanol fuel cell which uses a mixture of methanol and water as the fuel.
A typical direct methanol fuel cell is now explained with reference to FIG. 7.
FIG. 7 is a view schematically showing a direct methanol fuel cell 101 having a fuel electrode 104, an oxidizer electrode 106 and an electrolyte membrane 108 in a housing 102. A fuel, which is a mixture of methanol and water, is supplied by a fuel pump 110 from a fuel tank 109 to a fuel electrode chamber 112. The fuel supplied into the fuel electrode chamber 112 permeate into the fuel electrode 104 to react therewith, generating protons (hydrogen ions) and electrons as well as carbon dioxide.
Generally, a porous material is used for the fuel electrode 104, and the reaction at the fuel electrode 104 is taking place at a layer bearing a catalyst in the vicinity of an interface with the electrolyte membrane 108. The protons generated in the fuel electrode 104 permeate through the electrolyte membrane 108 to move to the oxidizer electrode 106, and the electrons flow from the fuel electrode 104 to the oxidizer electrode 106 via an external circuit (not shown). These electrons are used as an output of the fuel cell. The carbon dioxide is discharged from the fuel electrode 104 to the fuel electrode chamber 112, and discharged through an outlet port 121 together with unreacted fuel. The carbon dioxide and the unreacted fuel discharged through the outlet port 121 are recovered to the fuel tank 109, and the carbon dioxide is discharged through a discharge port 114 provided in the fuel tank 109.
Meanwhile, on the oxidizer electrode 106 side, oxygen is supplied to an oxidizer electrode chamber 118 by an oxygen compressor 116, and the oxygen is diffused from the oxidizer electrode chamber 118 into the oxidizer electrode 106. In the oxidizer electrode 106, oxygen reacts with protons diffused from the fuel electrode 104 to generate water. The generated water, normally transforming into steam, is discharged together with unreacted oxygen from the oxidizer electrode chamber 118 through an outlet port 120. In the example shown in FIG. 7, oxygen is used as the oxidizer. In addition, although lower in oxygen concentration, air may also be used as the oxidizer.
In the conventional direct methanol fuel cell, the mixture of methanol and water serving as the fuel is, as shown in FIG. 7, supplied to the fuel electrode chamber 112, permeates from the fuel electrode chamber 112 to the diffusion layer of the fuel electrode 104 to undergo a reaction at a catalyst-containing layer in the vicinity of the interface with the electrolyte membrane 108. Then, carbon dioxide, which is a reaction product, is discharged into the fuel electrode chamber 112, merging with supplied fuel and being discharged together with unreacted fuel through the outlet port 121. Fulfilling high-efficiency, stable power generation with a fuel cell involves efficient and stable fulfillment of the fuel supply and the discharge of carbon dioxide, which is a reaction product.
In this connection, a primary flow of the fuel supplied by the fuel pump 110 is fed into the fuel electrode chamber 112 before discharged through the outlet port 121 provided in the fuel electrode chamber 112. Because of this, a flow of the fuel within the porous material of the fuel electrode 104 that contributes directly to the reaction of the fuel electrode 104 is departed from the primary flow of the fuel within the fuel electrode chamber 112. Further, in the porous material of the fuel electrode 104, although a capillary action works, yet it is subject to constraints on the configuration or direction, making it difficult heretofore to efficiently and stably supply the fuel into the fuel electrode 104. This would also incur a difficulty in improving the output as a fuel cell and keeping power generation at high efficiency for long time. Furthermore, a pump for supplying the fuel with high pressure, when used, would incur an upsizing of the power supply unit, which makes it difficult to adopt the fuel cell as power supply particularly for portable equipment or the like.
JP 2002-175817 A shows that a fuel permeation member into which the fuel permeates is placed on a fuel passage for fuel supply so as to facilitate the fuel supply to the fuel electrode.
However, with the fuel cell described in JP 2002-175817 A, since the fuel is fed to the fuel electrode through permeation by a fuel permeation member, the fuel cell would be insufficient in reaction efficiency of the fuel at the fuel electrode, and therefore insufficient in power.